Petrology and geochemistry of Plio-Quaternary high-Nb basalts from Shahr-e-Babak area:Insights into post-collision magmatic processes in the Kerman Cenozoic Magmatic Arc

Abstract

Post-collision Pliocene-Quaternary basaltic rocks outcrop in the Kerman Cenozoic Magmatic Arc (KCMA) to the northwest and east of Shahr-e-Babak city. These porphyritic and vesicular basaltic rocks are composed essentially of clinopyroxene, olivine, and plagioclase. These basalts display alkaline affinity and negative Ta, Zr, Rb anomaly, but slightly negative Nb anomaly, relative to elements with similar compatibility, and positive Ba, K, Sr anomaly, suggesting their magma source related to subduction-accretion with implication of subducted slab derived components to the source. In the primitive mantle and chondrite normalized diagrams, these rocks show trace elements (except depletion in Nb, Ta) and Rare Earth Element (REE) patterns similar to the Ocean Island Basalts (OIB) and share trace and major element characteristics similar to High-Nb Basalts (HNBs). Geochemical analyses for major and trace elements suggest that the Shahr-e-Babak HNBs have undergone insignificant crustal contamination and minor olivine + Fe-Ti oxide ±clinopyroxene fractional crystallization. These HNBs derived from a partial melting (~5%) of garnet-peridotite mantle wedge, which have already metasomatized by overlying sediments, fluids, and adakitic (slab-derived) melts as major metasomatic agents in post-collision setting in the KCMA. We conclude that asthenospheric upwelling arising from slab break-off followed by the roll-back of subducting Neotethys slab also triggered metasomatized peridotite mantle wedge and caused its partial melting in the subduction zone.